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The acquisition of terrestrial, limb-based locomotion during tetrapod evolution has remained a subject of debate for more than a century.sup.1,2. Our current understanding of the locomotor transition from water to land is largely based on a few exemplar fossils such as Tiktaalik.sup.3, Acanthostega.sup.4, Ichthyostega.sup.5 and Pederpes.sup.6. However, isolated bony elements may reveal hidden functional diversity, providing a more comprehensive evolutionary perspective.sup.7. Here we analyse 40 three-dimensionally preserved humeri from extinct tetrapodomorphs that span the fin-to-limb transition and use functionally informed ecological adaptive landscapes.sup.8-10 to reconstruct the evolution of terrestrial locomotion. We show that evolutionary changes in the shape of the humerus are driven by ecology and phylogeny and are associated with functional trade-offs related to locomotor performance. Two divergent adaptive landscapes are recovered for aquatic fishes and terrestrial crown tetrapods, each of which is defined by a different combination of functional specializations. Humeri of stem tetrapods share a unique suite of functional adaptations, but do not conform to their own predicted adaptive peak. Instead, humeri of stem tetrapods fall at the base of the crown tetrapod landscape, indicating that the capacity for terrestrial locomotion occurred with the origin of limbs. Our results suggest that stem tetrapods may have used transitional gaits.sup.5,11 during the initial stages of land exploration, stabilized by the opposing selective pressures of their amphibious habits. Effective limb-based locomotion did not arise until loss of the ancestral 'L-shaped' humerus in the crown group, setting the stage for the diversification of terrestrial tetrapods and the establishment of modern ecological niches.sup.12,13. Analysis of humeri from fossils that span the fin-to-limb transition reveal that the change in the humerus shape is driven by both ecology and phylogeny, and is associated with functional trade-offs related to locomotor performance., Author(s): Blake V. Dickson [sup.1] , Jennifer A. Clack [sup.2] , Timothy R. Smithson [sup.2] , Stephanie E. Pierce [sup.1] Author Affiliations: (1) Museum of Comparative Zoology, Department of Organismic [...]

The known diversity of tetrapods of the Devonian period has increased markedly in recent decades, but their fossil record consists mostly of tantalizing fragments.sup.1-15. The framework for interpreting the morphology and palaeobiology of Devonian tetrapods is dominated by the near complete fossils of Ichthyostega and Acanthostega; the less complete, but partly reconstructable, Ventastega and Tulerpeton have supporting roles.sup.2,4,16-34. All four of these genera date to the late Famennian age (about 365-359 million years ago)--they are 10 million years younger than the earliest known tetrapod fragments.sup.5,10, and nearly 30 million years younger than the oldest known tetrapod footprints.sup.35. Here we describe Parmastega aelidae gen. et sp. nov., a tetrapod from Russia dated to the earliest Famennian age (about 372 million years ago), represented by three-dimensional material that enables the reconstruction of the skull and shoulder girdle. The raised orbits, lateral line canals and weakly ossified postcranial skeleton of P. aelidae suggest a largely aquatic, surface-cruising animal. In Bayesian and parsimony-based phylogenetic analyses, the majority of trees place Parmastega as a sister group to all other tetrapods. Three-dimensionally preserved fossils of Parmastega aelidae, a newly described tetrapod from the earliest Famennian (Late Devonian) of Russia, provide detailed insights into the morphology and palaeobiology of the earliest tetrapods., Author(s): Pavel A. Beznosov [sup.1] , Jennifer A. Clack [sup.2] , Ervins Luksevics [sup.3] , Marcello Ruta [sup.4] , Per Erik Ahlberg [sup.5] Author Affiliations: (1) Institute of Geology, Komi [...]

The construction of the vertebral column has been used as a key anatomical character in defining and diagnosing early tetrapod groups (1). Rhachitomous vertebrae2--in which there is a dorsally placed neural arch and spine, an anteroventrally placed intercentrum and paired, posterodorsally placed pleurocentra--have long been considered the ancestral morphology for tetrapods (13-6). Nonetheless, very little is known about vertebral anatomy in the earliest stem tetrapods, because most specimens remain trapped in surrounding matrix, obscuring important anatomical features (7-9). Here we describe the three-dimensional vertebral architecture of the Late Devonian stem tetrapod Ichthyostega using propagation phase-contrast X-ray synchrotron microtomography. Our scans reveal a diverse array of new morphological, and associated developmental and functional, characteristics, including a possible posterior-to-anterior vertebral ossification sequence and the first evolutionary appearance of ossified sternal elements. One of the most intriguing features relates to the positional relationships between the vertebral elements, with the pleurocentra being unexpectedly sutured or fused to the intercentra that directly succeed them, indicating a 'reverse' rhachitomous design (10). Comparison of Ichthyostega with two other stem tetrapods, Acanthostega (7) and Pederpes (8), shows that reverse rhachitomous vertebrae may be the ancestral condition for limbed vertebrates. This study fundamentally revises our current understanding (11) of vertebral column evolution in the earliest tetrapods and raises questions about the presumed vertebral architecture of tetrapodomorph fish (12,13) and later, more crownward, tetrapods., Jarvik (3,14,15) reassembled the vertebral column of the Devonian stem tetrapod Ichthyostega from 'imperfectly' preserved material. Each vertebral unit was thought to consist of a relatively straight neural spine; paired, [...]

The origin of tetrapods and the transition from swimming to walking was a pivotal step in the evolution and diversification of terrestrial vertebrates. During this time, modifications of the limbs--particularly the specialization of joints and the structures that guide their motions--fundamentally changed the ways in which early tetrapods could move (1-4). Nonetheless, little is known about the functional consequences of limb anatomy in early tetrapods and how that anatomy influenced locomotion capabilities at this very critical stage in vertebrate evolution. Here we present a three-dimensional reconstruction of the iconic Devonian tetrapod Ichthyostega and a quantitative and comparative analysis of limb mobility in this early tetrapod. We show that Ichthyostega could not have employed typical tetrapod locomotory behaviours, such as lateral sequence walking. In particular, it lacked the necessary rotary motions in its limbs to push the body off the ground and move the limbs in an alternating sequence. Given that long-axis rotation was present in the fins of tetrapodomorph fishes (5-7), it seems that either early tetrapods evolved through an initial stage of restricted shoulder (8, 9) and hip joint mobility or that Ichthyostega was unique in this respect. We conclude that early tetrapods with the skeletal morphology and limb mobility of Ichthyostega were unlikely to have made some of the recently described Middle Devonian trackways (10)., Early tetrapods, and the fish that gave rise to them, were originally interpreted as being terrestrially capable animals with load-bearing fins or limbs (11-13). Since the 1990s, however, new fossil [...]

The early tetrapod Crassigyrinus scoticus was a large aquatic predator known from the lower- to mid-Carboniferous (upper Tournasian to upper Visean/lower Serpukovian, approximately 350–330 Ma) of Scotland and Canada. Crassigyrinus is enigmatic in terms of its phylogenetic position due to its unusual morphology, which features a mixture of primitive and derived characters. Previous reconstructions, based on five incomplete and deformed specimens, have suggested a dorsoventrally tall skull with a short and broad snout, large orbits and external nares, and an extended postorbital region. In this study, we scanned four specimens using computed tomography and segmented imaging data to separate bone from matrix and individual bones from each other. Based on these data, we present a revised description of the upper and lower jaws, including sutural morphology and abundant new anatomical information. Damage was repaired and the skull retrodeformed to create a hypothetical three-dimensional reconstruction of the skull of Crassigyrinus that is dorsoventrally flatter than earlier reconstructions, yet still morphologically unique amongst early tetrapods. Overall skull shape, the size and distribution of the teeth, sutural morphology, and the specialized anatomy of the jaw joint and mandibular symphysis all suggest that Crassigyrinus was a powerful aquatic predator capable of hunting and subduing large prey. [ABSTRACT FROM AUTHOR]

The gap in our understanding of the evolutionary transition from fish to tetrapod is beginning to close thanks to the discovery of new intermediate forms such as Tiktaolik roseae. Here [...]

Author(s): Per Erik Ahlberg (corresponding author) [1]; Jennifer A. Clack [2]; Henning Blom [1, 2] Ichthyostega was the first Devonian tetrapod to be subject to a whole-body reconstruction [1, 2, [...]

Author(s): Sophie Sanchez (corresponding author) [1, 2]; Paul Tafforeau [2]; Jennifer A. Clack [3]; Per E. Ahlberg [1] The transition from fish to tetrapod was arguably the most radical series [...]